U.S. Pat. No. 4,856,465 discloses an internal combustion engine having control times which are varied via a rotational shift of a camshaft relative to a camshaft drive in dependence upon operating parameters. The rpm N of the internal combustion engine as well as the load signal T1 K*Q/N are exemplary of operating parameters. The load signal is determined in the computer for the injection and corresponds to the air throughput standardized to a stroke of the engine.
Conventional load determining methods measure the air mass flowing into the intake pipe (for example, with a hot-wire air-flow sensor) or utilize the throttle flap open angle in combination with the rpm or the intake pressure Ps as a measure for the load.
In the case where an air mass is measured, the air mass actually flowing into the cylinder can be determined with the aid of an air-mass balance for the intake pipe. The following applies for the change dMs of the air mass Ms in the intake pipe during the intake stroke of a cylinder: EQU dMs=Mdk-Mz (1)
wherein: Mdk is the air mass flowing in through the throttle flap during the intake stroke; and, Mz is the air mass inducted by the cylinder.
mMz can be approximately described by the relationship of intake pipe volume Vs and effective displacement (swept volume) Vh/Z*eta as delineated below by assuming that a portion (eta) of the displacement is charged with a fresh charge of air/fuel mixture. ##EQU1## wherein: Z is the number of cylinders and Vh is the displacement of the engine. The variable (eta) is also characterized as the volumetric efficiency. If (eta) does not change significantly, the following results for the change of the air charge of the cylinder dMz: ##EQU2##
The air mass Mdk flowing in through the throttle flap can be expressed as follows with the aid of the measured air flow Qm and the time duration dT of the intake stroke: EQU Mdk=Qm*dT (4)
The time duration dT follows from the engine rpm N and the number of cylinders Z: ##EQU3##
The change dMz of the air charge of the cylinder therefore follows: ##EQU4##
From equation (6), a recurrence equation can be derived for determining the air charge of the cylinder: EQU dMz=Mz(k)-Mz(k-1) (7)
and therefore: ##EQU5##
For internal combustion engines having a variable charge-cycle control, this computation rule, however, does not lead to optimal results under some circumstances. If, for example, the valve overlap (that is, the time span during which the inlet and outlet valves are opened simultaneously) is changed, then the exhaust-gas mass also changes which flows back from the exhaust manifold into the intake pipe during valve overlap. This exhaust gas displaces a portion of the fresh charge of air/fuel mixture in the cylinder and thereby causes a great change of the volumetric efficiency (eta). The preconditions for the derivation of equation (8) are therefore no longer present. The method up to now can therefore not correctly determine the air charge of the cylinder during transient operation which leads to intense fluctuations in the fuel/air ratio, especially during transient operation.
In the case of a measurement of the air pressure Ps in the intake pipe, the air charge of the cylinder can be computed by means of a simple straight-line equation as follows: EQU Mz=K*(Ps-Ps0) (9)
The load signal determination from throttle flap angle and the rpm is known and is a further example of load determination.
For engines having variable charge-cycle control, it has however been shown that the factor K and the offset Ps0 in equation (9) are greatly dependent upon the position of the camshaft and on the rpm. This method too does not provide an adequately precise result for displaceable camshafts.